1. Field of the Invention
The present invention relates to a photovoltaic cell which is typically used as a solar cell and a manufacturing method thereof.
2. Description of the Related Art
As an alternative energy source to a fossil fuel, a solar cell utilizing sunlight is attracting people's attention, and various studies have been conducted thereon. Prevailing solar cells that are now in practical use are those made of polycrystalline or amorphous silicon. However, these kinds of solar cells have problems that the economic costs and energy costs in a manufacturing process are high, and also a highly toxic material such as gallium or arsenic is used.
Japanese Unexamined Patent Publications Nos. SHO 58(1983)-188169, SHO 59(1984)-124772 and HEI 01(1989)-289173 disclose silicon type solar cells having different absorption spectra which are laminated on a single substrate. However, none of the solar cells disclosed in the above-mentioned publications could solve the above-mentioned problems.
On the other hand, Japanese Unexamined Patent Publication No. HEI 05(1993)-504023 (International Publication No. WO 9116719), Japanese Patent Publication No. 2664194 and International Publication No. WO 9405025 disclose new wet-type solar cells utilizing an optically induced electron movement of a metal complex.
These wet-type solar cells are constructed with a semiconductor electrode, a counter electrode and an electrolyte layer sandwiched between these electrodes. The semiconductor electrode includes an electrode and a semiconductor layer made of a photovoltaic raw material. A photosensitizing colorant having an absorption spectrum in a visible light region is adsorbed on a surface of the semiconductor layer.
In these wet-type solar cells, when the semiconductor electrode is irradiated with light, electrons are generated on this semiconductor electrode's side and moved via an electric circuit to the counter electrode. The electrons moved to the counter electrode are carried by ions in the electrolyte and return to the semiconductor electrode. This process is repeated to allow an electric energy to be taken out.
However, since the photosensitizing colorant as used has a limited light absorption range, it is not possible to effectively utilize sunlight in the range from visible light to near infrared light. Therefore, it is difficult to obtain such a high conversion efficiency as a silicon type solar cell can provide.
Also, International Publication No. WO 9405025 discloses the use of two colorants and a fluorescent brightener as spectroscopic sensitizing colorants on the surface of a metal oxide semiconductor. Use of two kinds of colorants allows more effective utilization of sunlight. However, if two kinds of colorants are allowed to be adsorbed simultaneously in a colorant adsorption step of a manufacturing process, it is difficult to quantitatively control the amount of adsorption, because the speed of adsorption on a metal oxide semiconductor differs depending on the kind of the colorants.